Modulation system



G. L. DAVIES MODULATION SYSTEM Filed May 6, 1942 May 18, 1943 2 Sheets-Sheet 1 LOAD Mi -P III MODULIATING VOLTAGE L OAD MODULATING VOLTAGE 8% [WWW I M y s, 1 4s DAVIES 2,319,53

MODULATION YSTEM Filed May 6, 1942 2 Shee ts-Sheet 2 LOAD MODULATING VOLTAGE LOAD MODULATING VOLTAGE grwe/M'o n GOMER L; DAVIES V a /M Patented May 18, 1943 MODULATION SYSTEM Gomer L. Davies, Woodside, Md., assignor to Washington Institute of Technology, Inc., Washington, D. 0., a corporation of Delaware Application May 6, 1942, Serial No. 441,950

13 Claims.

This application is a continuation in part of my application Serial No. 323,003, filed March 8, 1940, for Modulation system. V

This invention relates to transmission line modulation systems of the type generally disclosed in British Patent No, 485,959 and is intended to provide such a system which will not require the use of additional impedance transforming devices and in which practical and usually available transmission lines may be employed.

In applying the transmission line modulating system illustrated in Fig. 7 of the aforesaid British patent, and in Fig.' 1 of the drawings of this application, to voice and tone modulation, generally unsatisfactory operation has been obtained. In systems fortransmitting modulated radio frequency energy, it is necessary, in order to obtain maximum efl'iciency, to transfer all available power from the source to the load and simultaneously to modulate this power completely. The previously disclosed system is unsatisfactory in that it fails to perform both these functions simultaneously. It will be apparent that in order to achieve simultaneous performance of both of these functions it isessential that the impedance of line 2 of the prior art system illustrated in Fig. 1 of the annexed drawings, at its junction with line I, vary from a value which is very low in comparison with the impedance of line I to a value which is very high in comparison with the impedance of line I. In such systems in which the length of line 2 of Fig. 1 was made equal to an odd multiple of quarter wavelengths of the energy output of the source it was found that for the minimum obtainable value of the impedances of the plates of tubes T the impedance of line 2 at its junction with line I was still low enough to prevent maximum transfer of power to the load. Thus, while the load current could be completely modulated it was found to be impossible to transfer all of the available power from the source to the load. In such systems in which the length of line 2 of Fig. 1 was made equal to an even number of quarter wavelengths of the energy output of the source it was found that for the minimum obtainable value of the tube plate impedances the impedance of line 2 at its junction with line I was still high enough to permit a large current to flow in the load circuit. Thus, while the power in the load was as high as could be obtained it was impossible to obtain complete modulation of the load current. These efiects were found to be due to the fact that since the impedance of the modulating tubes T could only be Varied between infinity and a relatively high finite value, it was necessary to cause that part of transmission line 1 between the source and the connection to the line 2 to have an extremely low characteristic impedance and to cause transmission line 2, connecting the line I and the tubes T, to have an extremely high characteristic impedance in order to obtain effective power transfer to the load without requiring excessive grid voltage and current in the modulating tubes. Transmission lines having the required characteristic impedances are not available and it was first attempted to solve the problem by employing impedance transforming devices at the junction of lines I and 2. This proposed solution was found to be entirely unsatisfactory.

By the present invention I have solved this problem by providing means which transform the impedances of the modulating tubes sufliciently to permit the use of usual and available transmission lines or other known devices.

The invention is described hereinafter and illustrated in the accompanying drawings, it being understood that such description and drawings are merely illustrative of the invention, which is not limited thereby or otherwise than by the appended claims.

Referring to the drawings,

Fig. 1 is a circuit diagram of a transmission line modulating system according to the prior art;

Fig. 2 is a circuit diagram of a transmission line modulating system according to the present invention;

Fig, 3 is a representation of curves illustrating one aspect of the invention, and

Figs. 4 and 5 are circuit diagrams of modified forms of the invention.

In accordance with one form of my invention, which is illustrated in Fig. 2 of the drawings, the plates of the modulating tubes T are connected to a transmission line 3, the elements of which are bridged by a grounded movable tap 4. The transmission line I, which connects the source and the load, is connected to the line 3 by a second transmission line 2, the connection between lines 2 and 3 being made at such a point along line 3 between the short-circuiting tap 4 and the plates of the modulating tubesas to cause the impedance of the tubes to be transformed to such a value as to permit complete modulation of the load current simultaneously with maximum transfer of power to the load.

The line 3 is employed to tune the tube capacitances to resonance and line 2 is preferably connected to line 3 near the shorted end thereof, thus providing a sufficiently low impedance at such point of connection to permit the use of a practical and available value of characteristic impedance for lines I and 2. The length of line 2 is preferably made equal to any multiple of one-quarter of the wavelength of the energy output of the source and line 2 is preferably connected to line I at a'point which is spaced from the input end of line I by any multiple of one quarter of the wavelength of the energy output of the source.

In adjusting the system the modulating tubes T are tuned to resonance by adjustment of the short-circuiting tap t along line 3. The point of connection of line 2 to line 3 is then varied until the maximum and minimum impedances obtained at the junction of lines 2' and3f are of proper value to effectively provide a short-circuit and an open circuit at the junction of lines I and 2'as the-modulating tubes are operated. The distance A between the short-circuiting tap 4 and the junction of lines 2 and 3is principally a function of the impedances of lines I and 2 and, to a much less extent, of the impedance of the tubes. It'has been found that the distance A varies inversely as the impedance of line I and directly'as the impedance of line 2,

If-v desired, the connection of lines 2 and 3 may be fixed, and the effective electrical length of the section A of line 3 may be varied by varying the position of the tap 4, the line 3 being then tuned by a variable condenser 5 which is connected between the plates of the tubes.

In Fig. 4" of the drawings is disclosed a second embodiment of the invention in which the shortcircuitedtransmission line 3 ofFig. 2 is replaced by an inductance I2 which is adjustably coupled to an inductance I i which is connected by quarter-wave transmission line 2 across line I at a point spaced from the. input end thereof by any number of quarter wave-lengths. The inductance I2, I4 may be tuned by a variable capacity I6 which is connected across it. In this embodiment of the invention the variable inductance is adjusted until maximum and minimum impedances obtained at the inductance are of proper value to effectively provide a short circuit and an open circuit at the junction of lines I and 2 as the modulating tubes are operated. Alternatively, and as illustrated in Fig. 5, the inductances I 2, It may be short-circuited sections of inductively coupled transmission lines 20, 22 which are adjustable with respect to each other to vary the inductive coupling therebetween.

In the operation of the system according to any of the various embodiments of the invention the impedance of the modulating tubes is varied, by the modulating voltage applied to the grids, from infinity to a minimum value depending u on the type and number of tubes employed. The impedance of the tubes is transformed to a much lower value by the tuned circuit comprising the short-circuited line 3 of Fig. 2 or the adjustable inductance I 2, I 4 of Fig. 4 or 2B, 22 of Fig. 5 and the tube capacitances. this lower value depending upon the characteristic-impedances of lines I and 2. The impedance variation caused by operation of the tubes s transferred by line 2 to the junction of lines I and 2 and thus causes modulation of the. radio frequency energy supplied by the source to the load. If the length of line 2 is equal to an odd multiple of one quarter of the wavelength of the energy output of the source, a high negative bias on the modulating tubes, which is sufiicient for complete cut-off, produces maximum impedance at the junction of lines 2 and 3 of Fig. 2 or at the adjustable inductance I2, I4 of Fig. 4 or 20, 22 of Fig. 5 and minimum impedance at the junction of lines I and 2, thus producing minimum or zero load current. As the bias on the modulating tubes is increased positively, the load current increases. If the length of line 2 is equal to an even multiple of one quarter of the wavelength of the energy output of the source, cut-off bias on the modulator tubes corresponds to maximum load current, and increasing the bias positively causes decreasing. load current.

Inasmuch" as the length of line 2 may be equal to an even multiple of one quarter of the wavelength of the energy output of the source, the length of this line may, if desired, be made equal to zero; However, the use of line: 2 is preferred in practical use as it makes the system more flexible and permits adjustments to be made more readily.

In Fig. 3 of the drawings are shown curves illustrating the variation in load current with variation in grid bias as the junction of lines 2 and 3 is moved to difierent positions with respect to the short-circuiting tap 4. It will be seen from these curvesthat, with constant grid voltage, the load current decreases as A is increased. Similar curves may be plotted to show the variation in load current with variation in grid bias as the coupling between the inductances of Figs. 4 and 5 is'varied. Such curves would also show that with constant grid voltage: the load current decreases as the coupling between the inductances of Figs. 4 and 5 increases.

While I. have described and illustratedcertain embodiments of my invention it will. be apparent to those skilled in the art that other embodiments thereof may be made without departing in any way from the scope of the invention, for the limits of which reference must be had to the appended claims.

What is claimed is:

1. An. electrical system comprising source of radio frequency energy, a load circuit, a transmission line connecting the source and the load circuit, a variabl impedance comprising vacuum tubes, a source of cyclic control signal for operatin said tubes to cyclically vary the impedance thereof, a second transmission line having its conducting elements respectively connected to the anodes of said tubes, a short-circuiting tap connecting the conducting elements of said second line and being adjustable along the length thereof, and a third transmission line connected at its one end across the first transmission line and at its other end being connected across the second transmission line for adjustment along the length thereof.

2. An electrical system comprising a source of radio frequency energy, a load circuit, a transmission line connecting the source and the load circuit, a variable impedance comprising vacuum tubes, a source of cyclic control signal for operating said tubes to cyclically vary the impedance thereof, a short-circuited transmission line having its conducting elements respectively connected to the anodes of said tubes, and a third transmission line connected at its one end across the first transmission line and at its other end line for adjustment along the length thereof.

3. An electrical system comprising a source of radio frequency energy, a load circuit, a transmission line connecting the source and the load circuit, a controllable variable impedance comprising vacuum tubes, a source of cyclic control signal for operating said tubes to cyclically vary the impedance thereof, a short-circuited transmission line connected in parallel to the anodes of said tubes, and a third transmission line connected at its one end across the first transmission line and at its other being being connected across the second transmission line for adjustment along the length thereof.

4. A system for modulating radio frequency energy comprising a source of energy and a load circuit connected thereto by a transmission line, a variable impedance for modulating the energy passed through said line, and means for transforming the impedance of the variable impedance, said means comprising a short-circuited transmission line connected in parallel with the variable impedance and means connecting the line between the source and the load across the short-circuited transmission line between the ends thereof.

5. A system for modulating radio frequency energy comprising a source of energy and a load circuit connected thereto by a transmission line, a variable impedance for modulating the energy passed through said line, and means for transforming the impedance of the variable impedance, said means comprising a short-circuited transmission line'connected in parallel with the variable impedance and a transmission line connecting the line between the source and the load across the short-circuited transmission line between the ends thereof.

6. An electrical system comprising a source of radio frequency energy, a load circuit, a transmission line connecting the source and the load circuit, a controllable variable impedance comprising vacuum tubes having anodes and cathodes, a source of cyclic signal for operating said tubes to cyclically vary the impedance thereof, a short-circuited transmission line connected in parallel to the anodes of said tubes, a third transmission line connected at its one end across the first transmission line and at its other end being connected across the second transmission line at points removed from said anodes and from the short-circuit thereof.

7. A system for modulating radio frequency energy supplied by a source of energy to a load connected thereto by a transmission line, comprising a variable impedance for modulating the energy passed through said transmission line, and

means for transforming the impedance of the variable impedance comprising an inductance connected across the variable impedance and connections between the inductance and the transmission line, said connections being adjustable with respect to the inductance to selectively include between the transmission line and the variable impedance all or a part of the inductance.

8. A system for modulating radio frequency energy supplied by a source of energy to a load connected thereto by a transmission line, comprising a plurality of variable impedances for modulating the energy passed through said transmission line, and means for transforming the impedance of said variable impedances comprising an inductance connected across said variable impedances and connections between the inductance and the transmission line, said connections being adjustable with respect to the inductance to selectively include between the transmission line and the variable impedances all or a part of the inductance.

9. A system for modulating radio frequency energy supplied by a source of energy to a load through a transmission line, comprising a plurality of variable impedances, means for connecting said impedances to the source and for transforming the impedance of said variable impedances comprising'an inductance connected across said variable impedances and a second inductance connected at its input end to said transmission line at a point spaced from the source by any number of quarter wave lengths of the energy supplied by the source and at its other end being adjustably and inductively coupled to the first inductance.

10. A system for modulating radio frequency energy supplied by a source of energy to a load through a transmission line, comprising a variable impedance, means for connecting said impedance to the sourceand for transforming the impedance of said variable impedance comprising an inductance connected across said variable impedance and a second inductance connected at its input end to said transmission line at a point spaced from the source by any number of quarter wave lengths of the energy supplied by the source and at its other end being adjustably and inductively coupled to the first inductance.

11. A system for modulating radio frequency energy supplied by a source of energy to a load through a transmission line, comprising a variable impedance, means for connecting said impedance to the source and for transforming the impedance of said variable impedance comprising an inductance connected across said variable impedance, a transmission line connected at its input end to said first transmission line at a pointspaced from the source by any number of quarter wave lengths of the energy supplied by the source and at its other end being adjustably coupled to said inductance.

12. A system for modulating radio frequency energy supplied by a source of energy to a load through a transmission line, comprising a plurality of variable impedances, means for connecting said variableimpedances to the source and for transforming the impedance of said variable impedances comprising an inductance connected across said variable impedances, a

transmission line connected at its input end to said first transmission line at a point spaced from the source by any number of quarter wave lengths of the energy supplied by the source and at its other end being adjustably coupled to said inductance.

13. A system for modulating radio frequency energy supplied by a source of energy to a load through a transmission line, comprising a variable impedance comprising a plurality of vacuum tubes, means for connecting said vacuum tubes to the source and for transforming the impedance of said variable impedance comprising an inductance connected across said vacuum tubes, and a second inductance connected at its input end to said transmission line at a point spaced from the source by any number of quarter wave lengths of the energy supplied by the source and at its other end being adjustably coupled to the first inductance. I

GOMER L. DAVIES. 

